It can be stated that in an EDM process, electric energy is furnished from the power supply in the form of discrete electrical pulses across the machining gap filled with a liquid dielectric to effect a succession of electrical discharges between the tool electrode and the workpiece to remove stock from the latter. Each individual discharge strikes that area of the workpiece juxtaposed with the tool electrode on one minute localized zone or another, the zone being impulsively melted and/or vaporized and mechanically dislodged from the workpiece area by the impulsive discharge pressure. Successive and repetitive discharges are used to sweep the localized stock dislodgment or removal action over the entire workpiece area and result in the formation of cumulatively overlapped discharge craters thereon. As stock removal proceeds, the tool electrode is advanced relatively towards the workpiece by servo feed means adapted to maintain the machining gap spacing substantially constant and thereby to allow stock removal discharges to be successively created.
Electrical discharge machining (EDM) processes are generally divided into three categories, sinking-type EDM, scanning-type EDM and traveling-wire EDM. In this sinking-type EDM process, the tool electrode is a formed solid electrode designed to from a cavity complementary in shape thereto in a workpiece. In this process, the workpiece is immersed in the machining liquid commonly constituted by a hydrocarbon liquid such as kerosene. A worktank is used to retain the hydrocarbon machining liquid and the workpiece immersed therein and positioned sufficiently below the surface of this liquid in the worktank. The tool electrode is commonly formed with one or more fluid passages therein through which the machining liquid is supplied into the machining gap. Alternatively or in addition, one or more nozzles are disposed in the region of the tool electrode or the workpiece and used to direct the machining liquid into the machining gap. It has been recognized that this process entails a danger of fire because of the inflammability of the hydrocarbon liquid. When electrical discharges effected through the hydrocarbon liquid are exposed to air, fire tends to develop and may result in property damage. Furthermore, the hydrocarbon liquid upon decomposition by electrical discharges produces gases and mists and tends to pollute the environmental atmosphere. While the danger of fire may be alleviated by adding certain chemicals to the hydrocarbon liquid, this adds to cost and may result in a significant reduction in the machining efficiency.
In the scanning-type EDM, the tool electrode is a rod or like electrode having a relatively simple or "generic" machining contour and a two- or three-dimensional relative displacement is effected between the generic electrode and the workpiece to yield a desired shaped configuration in or on the workpiece corresponding to the path of the relative displacement.
In the traveling-wire EDM process, the tool electrode is a thin, continuous wire which is axially transported to continuously traverse the workpiece exposed to air. The machining gap formed between the traveling wire electrode and the workpiece needs to be consecutively flushed with the machining liquid which, however, with the workpiece always exposed to air, can in no way be served by an inflammable hydrocarbon liquid as in the sinking-type EDM process. Thus, in the traveling-wire EDM process, it has been the common practice to employ a water liquid as the machining liquid. It has been found that water is much greater in efficiency than hydrocarbon liquids in cooling of the workpiece and in cooling the discharge spots and the particles removed therefrom.
With the traveling-wire process gaining increasing popularity, attempts have been made to apply a water liquid which found its sole use in that process to the sinking-type and scanning-type processes. It has been found, however, that the water liquid when used in these latter processes has the significant disadvantage that a mirror-finish machined surface as required in a finish or ultra-fine machining range are not obtainable. In addition, it has been found that it is unsuitable for use with "no-wear" or "low-wear" machining modes which require the erosive wear of the tool electrode to be minimized. Moreover, it has been observed that with the tool machining area reduced, say, to less than several hundred square millimeters, the water machining liquid seldom offers the required machining stability and efficiency in the sinking-type and scanning-type processes.
These deficiencies of the water liquid give rise to problems in the traveling-wire process as well. Thus, the cutting tends to be unstable for a thicker workpiece or with a thicker wire electrode. The result is a breakage of the wire electrode.